The present invention relates to preparation of a herbicidal formulation useful in agriculture and in other situations where control of weeds or other vegetation is desired. In particular, it relates to a process for preparing a herbicidal composition containing as an active ingredient N-phosphonomethylglycine (glyphosate) in the form of the ammonium salt thereof. According to the process of the present invention, particulate glyphosate acid, ammonia, and water react to form an ammonium glyphosate paste, and as part of the process a surfactant is added to the reaction mixture to increase the reaction rate and enhance the physical properties of the resulting product.
Glyphosate herbicides, especially herbicides comprising a water-soluble salt of glyphosate, are well known. The monoammonium salt of glyphosate is disclosed as a useful herbicide for example in U.S. Pat. No. 4,405,531 issued to Franz. Unless the context demands otherwise, xe2x80x9cammonium glyphosatexe2x80x9d herein refers to the monoammonium salt of glyphosate, which has the chemical formula 
Ammonium glyphosate may be produced in granular form by a continuous process in which glyphosate acid is intimately mixed with ammonia. For example, U.S. Pat. No. 5,070,197 issued to Chin et al. disclose a continuous process in which a Bronsted acid is mixed with a Bronsted base in an extruder to produce a salt. Chin et al. list glyphosate acid as one of the potential Bronsted acids that may be used and ammonia as one of the potential bases that may be used. According to Chin et al., the reaction may be carried out as a solid-state reaction essentially without addition of xe2x80x9cextraneous solventxe2x80x9d such as water, although it is stated that a small amount of water (usually about 4% by weight) may be optionally added upstream for xe2x80x9cinitial lubricityxe2x80x9d. An acid-base reaction is said to occur in the extruder, forming a dry salt which is extruded to form granules.
Ammonium glyphosate is particularly useful, however, in the preparation of dry glyphosate herbicide formulations which contain one or more surfactants in addition to the glyphosate salt. Surfactants are important components of glyphosate formulations because when a glyphosate formulation is diluted, dissolved or dispersed in water for application by spraying to foliage of plants, the surfactants enhance the herbicidal effectiveness of the glyphosate spray by assisting in: (i) the retention of droplets of the spray by the foliage; (2) the adhesion of the spray droplets to the foliar surface; and, (3) the penetration of the glyphosate through the hydrophobic cuticle that covers the foliar surface. Commercially available dry ammonium glyphosate formulations containing one or more surfactant(s) include, for example, Roundup(copyright) Dry, Roundup(copyright) Max and Rival(copyright) herbicides, marketed by Monsanto Company in several countries.
Processes for producing dry ammonium glyphosate formulations typically require combining one or more surfactant(s) with a dry ammonium glyphosate salt and granulating the mixture to form dry ammonium glyphosate granules having the desired concentration of surfactant. Numerous granulation processes have been disclosed that are suitable for preparing water-soluble or water-dispersible granules of ammonium glyphosate with a liquid surfactant. One such process is pan granulation. However, a more widely used granulation process for a dry ammonium glyphosate formulation is extrusion granulation. British Patent No. 1 433 882 (xe2x80x9cthe ""882 patentxe2x80x9d) generally describes an extrusion granulation process, and the granulation of dry ammonium glyphosate formulations are typically carried out in a similar process except that the primary active ingredient, namely ammonium glyphosate, is water-soluble rather than water-insoluble as in the process of the ""882 patent. In this process, ammonium glyphosate is mixed with surfactant and a small amount of water to form an extrudable wet mix, which is then extruded to form strands of extrudate that break spontaneously at the point of extrusion or shortly thereafter to form short cylindrical granules, which are then dried.
The desired amount of surfactant for delivering the desired herbicidal effectiveness may be typically in the range of about 0.1 to about 1 part by weight of surfactant per part by weight of glyphosate, expressed as acid equivalent (a.e.), depending on the particular surfactant used. Ammonium glyphosate formulations having such a high concentration of surfactants may be difficult to achieve depending on the degree to which the surfactant is absorbed or adsorbed by the ammonium glyphosate. That is, frequently such high surfactant concentrations do not sufficiently absorb and/or adsorb to the ammonium glyphosate thereby causing the resulting formulation to become sticky, have a tendency to cake, or lack good pouring or flow properties. Thus, the absorbency and adsorbency properties of the ammonium glyphosate particles are especially important.
The process by which ammonium glyphosate is prepared has been found to affect the absorptive and/or adsorptive properties of the ammonium glyphosate particles with respect to a liquid surfactant. Recently, processes for producing a dry glyphosate powder having high absorptive and/or adsorptive characteristics suitable for producing ammonium glyphosate formulations having a high concentration of surfactant have been discovered. For example, U.S. Pat. No. 5,614,468 (xe2x80x9cthe ""468 patentxe2x80x9d), Kramer et al. disclose processes wherein particulate glyphosate acid is reacted with aqueous ammonia to produce a dry glyphosate powder having a moisture content of no greater than about 2% by weight of water. Similarly, U.S. Pat. No. 5,633,397 (xe2x80x9cthe ""397 patentxe2x80x9d), Gillespie et al., disclose a process for producing ammonium glyphosate powder wherein particulate glyphosate acid is reacted with gaseous anhydrous ammonia to produce a dry powder. As shown in Example 1 of the ""397 patent, a dry powder having a moisture concentration of about 1% by weight may be produced by the process disclosed therein.
Both the process disclosed in the ""468 patent and the process disclosed in the ""397 patent produce a dry ammonium glyphosate powder capable of adsorbing and/or absorbing high concentrations of surfactant. However, dry solid-state reaction processes wherein the moisture content is suppressed to ensure the formation of a dry product may be more difficult to control than a process wherein glyphosate and ammonia are reacted in an aqueous medium. The exothermic nature of the reaction between ammonia and glyphosate acid gives rise to a need for the dissipation of heat, which can present problems. Since solid-state processes typically exhibit poor heat transfer properties, the dissipation of heat may become problematic.
PCT Application No. WO 01/08492, Massmann et al. disclose a process for producing ammonium glyphosate which avoids some of the difficulties encountered with processes carried out in a dry solid-state medium. Particulate glyphosate acid, ammonia in the form of either gaseous anhydrous ammonia or aqueous ammonia and water are added to a reactor to from a reaction mixture having a moisture content of about 10% to about 25% by weight. The glyphosate acid and ammonia react and a portion of the water is evaporated to produce a downstream processable ammonium glyphosate paste having a moisture content of about 5% to about 20% by weight. A surfactant is then mixed with the ammonium glyphosate paste prior to extrusion granulation, in part to condition the paste to improve the extrudability of the paste, but primarily to improve the herbicidal efficacy of the final ammonium glyphosate granules. According to Massmann et al., adding surfactant prior to or during the reaction process is detrimental to the process; thus, the entire amount of surfactant is added to the ammonium glyphosate paste after the entire reaction has been completed and preferably after the ammonium glyphosate paste has been cooled to less than 70xc2x0 C.
Among the objects of the present invention is the provision of a continuous process for preparing a moist ammonium glyphosate paste; the provision of such a process wherein one or more adjuvant(s) are added to the reaction mixture to improve the rate of formation of the ammonium glyphosate paste and/or reduce the flow resistance of the reaction mixture; the provision of such a process wherein an additional quantity of adjuvant is subsequently added to the ammonium glyphosate paste to form an extrudable ammonium glyphosate mixture; the provision of such a process wherein the extrudable ammonium glyphosate mixture is extruded to form ammonium glyphosate granules; the provision of an improved process for extruding ammonium glyphosate having increased throughput; and, the provision of a process for extruding ammonium glyphosate having reduced power requirements.
Briefly, therefore, the present invention is directed to a process for preparing an ammonium glyphosate paste, comprising mixing in a reactor (i) particulate glyphosate acid, (ii) ammonia, (iii) water and (iv) adjuvant to form a reaction mass wherein the total amount of water added to the reaction mass is from about 10% to about 25% by weight of all of the glyphosate acid, ammonia, water and adjuvant added to the reactor.
The present invention is further directed to a continuous process for preparing an ammonium glyphosate paste, comprising continuously feeding to a reactor (i) particulate glyphosate acid, (ii) ammonia, (iii) adjuvant, and (iv) water, thereby causing a reaction of the glyphosate acid and ammonia generating heat which causes partial evaporation of the water, and forms an ammonium glyphosate paste having a moisture content of less than about 20% by weight. In a preferred embodiment, the moisture content will fall within the range of from about 2% to about 15% by weight.
The present invention is further directed to a continuous process for preparing a granular herbicidal composition, comprising continuously feeding (i) particulate glyphosate acid, (ii) ammonia, (iii) adjuvant, and (iii) water to form a reaction mass in which ammonium glyphosate is formed, wherein the amount of water fed is from about 10% to about 25% by weight of all materials being fed to the reactor. The reaction of glyphosate acid and ammonia generates heat, causing the evaporation of water such that the water content of the reaction mass is reduced to from about 2% to about 10% to form an ammonium glyphosate paste. The reaction mass is continuously conveyed to the product discharge region of the reactor and as part of the process, the adjuvant (i) increases the rate at which the glyphosate acid and ammonia react to form ammonium glyphosate and/or (ii) reduces the flow resistance of the reaction mass as it is being conveyed to the product discharge region of the reactor.
The present invention is further directed to a process for preparing a dry granular herbicidal composition, comprising continuously feeding (i) particulate glyphosate acid, (ii) ammonia, (iii) adjuvant, and (iii) water to form a reaction mass in which ammonium glyphosate is formed, wherein the amount of water fed is about 10% to about 25% by weight of all of the glyphosate acid, ammonia, adjuvant and water added to the reactor. The water content of the reaction mass is reduced to form an ammonium glyphosate paste using the heat of reaction of the glyphosate acid and ammonia to cause the evaporation of water. The reaction mass is continuously conveyed to the product discharge region of the reactor. If the ammonium glyphosate paste has a moisture content greater than about 15% by weight, heat and/or vacuum may be applied to reduce the moisture content of the composition to from about 2% to about 15% by weight. One or more adjuvant(s) in a weight ratio of total adjuvant to ammonium glyphosate of about 1:20 to about 1:2 are added to the ammonium glyphosate paste to form an extrudable ammonium glyphosate mixture. The extrudable ammonium glyphosate mixture is continuously fed to an extruder having a screen through which the ammonium glyphosate is extruded to form extrudate strands that break to form moist coherent granules. As part of the process, the adjuvant: (i) increases the rate at which the glyphosate acid and ammonia react to form ammonium glyphosate; (ii) reduces the flow resistance of the reaction mass as it is being conveyed to the product discharge region of the reactor; and/or, (iii) reduces the flow resistance of the extrudable ammonium glyphosate mixture. The granules may then be dried to produce the dry granular composition and/or further processed to classifying the granules to remove or recycle granules, fragments of granules and aggregates of granules that are outside a desired size range.